Station circuit



Oct. 26, 1943. o. A. SHANN STATION CIRCUIT Filed July 29, 1942 2 Sheets-Sheet l INVENTOR 0. A. SHANN ATTORNEY Oct. 26, 1943. 0. A SHANN STATION CIRCUIT Filed July 29, 1942 2 Sheets$heet 2 INVENTOR By 0 A. SHANN ATTOPNFY Patented Oct. 26, 1943 Oscar A. Shann, Short Hills, Bell Telephone Laboratories,

NI 1., assignor to Incorporated, New

York, N, Y., a corporation of New York Application July 29, 1942, Serial No. 452,732

5 Claims.

This invention relates to communication systems and particularly to station arrangements to .permit emergency operation.

The object of the invention is to provide means whereby a subscriber under normal conditions is required to perform a certain operation such as to deposit a coin, may dispense with this requirement in ordersto make an emergency call.

The invention is based on the use of a thermistor, a device whose resistance changes rapidly due to the thermal effect of current flowing therethrough. It is possible to construct a thermistor having an initial resistance 50 high that bridged across a telephoneline it will not interfere with the normal dialing operation. However, with current flowing through this device it quickly changes its resistance to a value low enough to void the effect of the dialing at the distant receiving end. With such a device bridged across the dialing contacts it can be arranged so that but a single digit may be transmitted, but since this is all that is necessary for an emergency call an arrangement of this naturewill be useful.

In accordance with the present invention a circuit arrangement is provided in which a thermistor is normally bridged across the dialing contacts of a telephone to prevent the unauthorized use of the telephone. When a coin is inserted in the proper receptacle then this connection will be broken so that full normal use may be made of the telephone. In addition a similar thermistor may be bridged across the switchhook contacts to prevent simulated dialing by the rapid manipulation of the switchhook.

In accordance with a variation of the invention the thermistor may be provided with a separate heating unit controlled by other circuit operations to prevent the avoidance of the effect produced by the simple form by the use of unduly long interdigit pauses in an effort to allow the simple form of thermistor to cool off between trains of transmitted impulses.

A feature of the invention is a means responsive to. a limited use of a dial to render further use thereof ineffective.

Another feature of the invention is the use of a negative resistance at one end of a line to render a relay at the other end thereof unresponsive to more than a limited number of interruptions of the line circuit.

Other features will appear hereinafter.

The drawings consist of two sheets having figures, as follows:

Fig. 1 is a circuit diagram showing a substation circuit embodying the present invention and a schematic showing of the cooperating central oflice circuits;

Fig. 2 is. a variation of the substation circuit of Fig. 1 showing the use of an indirectly heated thermistor;

Fig. 3 is a circuit diagram practically the same as that of Fig. 1 except that a different arrangement is employed to originate a call. This is i1- lustrated in Fig. 4 which shows a skeletonized perspective view of a button, the coin-trigger and the contacts operated thereby;

Fig. 5 is another circuit diagram similar to that of Fig. 1, in which a two step coin trigger is employed;

Fig. 6 is a skeletonized perspective view of the two step coin trigger; and

Fig. 7 is a circuit diagram showing the use of a thermistor to prevent the simulation of dialing by the manipulation of the switchhook.

Reference is made to Patent 2,235,648 granted to Shann et al., March 18, 1941 which shows a coin collector upper housing which carries a dial and the transmitters used for detecting the passage of a coin. This apparatus is then connected with the rest of the substation apparatus and circuits by a set of contacts which cooperate with a corresponding set when the coin collector housing is placed in proper position. Patent 2,251,- 073, granted to O. A. Shann, July 29, 1941 is referred to as showing in detail the coin trigger arrangement usually employed. Patent 2,132,241, granted to H. Hovland, October 4, 1938 is referred to as showing dial central office circuits operable from a coin box station such as that herein disclosed.

In Fig. 1 a coin disposal magnet l is shown. Above this is a coin trigger 2, which on operation will operate the contacts to close a circuit between the contacts A and B and to open a circuit between contacts C and D. Upon the deposit of a coin these contacts will be operated-as stated and the substation will then be enabled for regular service. Thereafter, by the use of the dial 3, the various switches at the central office may be set and a connection established to substation A. Through the connection established from ground contact B, contact A, coin magnet 1, contacts 3 and 4 of switchhook 6, contacts 5 of the housing contact set, contacts 6 and 5 of the dial 3, transmitters l and 8, contacts 5 of the housing contact set, transmitter of the handset 9, left-hand winding of the induction coil 10, ring of, the line, left-hand armature of cut-oii relay ance with well-known practice. Thereafter the central ofiice apparatus may be set under dial control in the usual manner.

In case an emergency call is to be made, one involving usually the dialing oi but a single digit,

without the deposit of a coin, then the subscriber may depress key II, which will perform the same function as the establishment of contact between the springs A and. B. However, since no .coin has been deposited, the thermistor H is in shunt oi the pulsing contacts I and I of the dial 3. The thermistor is a device which has a resistance at ordinary room temperature in the order of one hundred thousand ohms. However, with a small amount of current flowing therethrough, it becomes heated and it resistance falls to something in the order of one to two hundred ohms. If an attempt is now made to dial, on each opening 01' the dial pulsing contacts, current will fiow through the thermistor It. The circuit conditions may be such that a single digit of ten pulses may be dialed without interference, but it much more than this is attempted, the resistance oi the thermistor becomes so-low that the pulse relay in the central ofilce will fail to respond. Hence an emergency call requiring the dialing of one digit may be made but a regular call 01' four or six or seven digits cannot be made without the deposit of a coin to break the connection between the springs C and D.

Fig. 2 shows a substation circuit similar in detail to that of Fig. l with the exception of the wiring of the thermistor. The line conductors l8 and I3 may be connected in a manner similar to the line conductors l8 and II, respectively, of Fig. 1 to the central ofilce apparatus shown to the right in Fig. 1. The thermistor "has a resistance element 2| which is heated by a heating element 22. When the dial 23 is moved oil normal, a circuit may be traced from the line conductor l9, through the left-hand winding oi induction coil 2|, heating coil 22, springs 3 01 the housing contacts. dial springs 3, I, I and 6, springs 5 01' the housing contacts, springs land 3 of the switchhook 25 to line conductor l3. Thus during the off-normal periods of the dial, the thermistor 20 is heated to lower its resistance and thus frustrate any attempt to dial a suiiicient number of digits to establish anything more than an emergency connection without the deposit of a coin.

In Fig. 3 the circuit arrangement is like that of Fig. l with the exception that the button [3 is eliminated and in its place a button 28 is employed. This is shown in detail in Fig. 4. When the button 26 is pressed, it engages an arm 21 formed on the piece with the coin trigger 28. The button 26 will not move this piece as far as a coin but will allow the latch 29 to drop down the first of two steps far enough to bring the springs A and B together but not far enough to break the contact between springs C and D. In this manner the ground for operating the line relay i2 is 'provided but the thermistor 30 is left in circuit.

Figs. 5 and 6 show another two step arrangement in which a coin trigger having two trigger arms is employed in much the same manner. In this arrangement it is intended that a coin may be deposited to make an emergency call, which coin may be returned by an operator or the normal functioning of the machine switching apasses-2a paratus. For making a normal call, the deposit oi two coins is required.

Thedcpositoithefirstcoinwillstrike the trigger arm 3| and cause a circuit to be established between the springs A and 3 while leaving the springs C and D still in contact with each other so that the thermistor 33 is left eil'ectively in circuitl Thus by the deposit of a single coin, the apparatus may be conditioned to make an emergency call.

In the normal operation of this apparatus, two coins are required to be deposited. The first causes the operation described and leaves the trigger arm 32 in the cointrack so that the second coin strikes this arm 32 and completely opcrates the contacts, opening the circuit between contacts C and D for normal operation.

In Fig. 7 a substation circuit is shown in which the thermistor ll plays the same role as in each of the other previously described figures. Any one of the previously described two step means for starting the call may be used. such as the two coin trigger of Fig. 6. In this case another pair of springs E and F are closed by the first coin so that a thermistor 35 is bridged across the contacts 3 and 4 01' the switchhook 38 to prevent an attempt at simulated dialing. Even if the thermistor is heated by such an attempt, it will cool oil-during the conversational period so that when the handset is hung on the switchhook at the end 01' the conversation, the line circuit will be effectively opened long enough to cause the central ofllce apparatus to properly work the coin disposal magnet 31 which will in turn release the springs E and F and thus open the circuit of thermistor II.

What is claimed is:

tem, including an impulse transmitter having a set of impulse contacts, a thermistor having an initially high resistance bridged across said contacts and subscriber controlled means for eiiectively excluding said thermistor from said circuit.

2. A station circuit for a communication system, including an impulse transmitter having a set of impulse contacts normally closed, a device bridged across said contacts having a normal high resistance value, said device being responsive to current flowing therethrough during operation of said contacts to change its resistance to a value low enough to render theoperation thereof ineffective and subscriber controlled means for opening the circuit of said device.

1. A station circuit for a communication sys- I 3. In a telephone system, a line, a station at I one end of said line, said station including an impulse transmitter, a source 01' current and an impulse relay responsive to said transmitter at the other end of said line, a negative resistance device bridged across said transmitter responsive to current from said source to change its resistance from a high to a low value, said relay being responsive to said transmitter while said resistance is at said high value and unresponsive to said transmitter while said resistance is at said low value and means at said station responsive to the deposit of a coin for excluding said negative resistance from said station circuit.

4. In a telephone system, a line, a station at one end of said line, said station including an impulse transmitter, a source of current and an impulse relay responsive to said transmitter at the other end of said line, a negative resistance device bridged across said transmitter. thermal means responsive to the operation of said transmitter for heating said device to change its resistance from a high to a low value, said relay being responsive to said transmitter while said resistance is at said high value and unresponsive to said transmitter while said resistance is at said low value and coinoperated means at said station for excluding said negative resistance from said station circuit.

5. In a telephone system, a line, a station at one end of said line, said station including an impulse transmitter and a set of switchhool: contacts, a source of current and an impulse relay responsive to the operation of said transmitter and the operation of said switchhook contacts, a negative resistance device bridged across said transmitter responsive to current from said source during the open periods of said transmitter to change its resistance from a high to a low value, a similar negative resistance device bridged across said switchhook contacts responsive to current from said source during the open periods thereof to change its resistance from a high to a low value, said relay being responsive to interruptions of said line circuit by said transmitter and said switchhook contacts while said resistances are at said high value and unresponsive to said interruptions while said resistances are at said low value and means for effectively excluding said negative resistance devices from said station circuit.

OSCAR A. SHANN. 

